1. Field of the Invention
The present invention relates generally to a protein tyrosine kinases and specifically to JAK3, a novel protein tyrosine kinase of the JAK family.
2. Description of Related Art
Proliferation and differentiation of hematopoietic cells is dependent upon the binding of hematopoietic growth factors and cytokines to their respective cell surface receptors (Cross, et al., Cell, 64:271, 1991; Ogawa, M., Blood, 81:2844, 1993; Heimfeld, S., et al., Proc. Natl. Acad. Sci. USA, 88:9902, 1991). Some of these receptors transduce the signal at the cell surface to the cytoplasm through the activation of a tyrosine kinase domain in the cytoplasmic portion of the receptor (e.g., CSF1, c-kit, STK-1/FLT3/FLK2-) (Boyle, W. J., Current Opinion in Oncology, 4:156, 1992, Chiba, T., et al, Nature, 362:646, 1993, Schlessinger, J., et al., Neuron, 9:383, 1992, Ullrich, A. and Schlessinger, J., Cell, 61:203, 1990). Another group of hematopoietic receptors lack intrinsic kinase catalytic domains (e.g., IL-3, GM-CSF, G-CSF, and EPO receptors) (Miyajima, A., et al., Blood, 82:1960, 1993, Fukunaga, R., et al., EMBO, 10:2855, 1991, Wojchowski, D. M., et al., Stem Cells, 11:381, 1993), however, upon binding of their ligands, these receptors activate protein tyrosine phosphorylation of second messengers and the subsequent signal pathways to the cell's nucleus (Kishimoto, T. et al, Science, 258:593, 1992, Stahl, N., et al., Cell, 74:587, 1993).
Tyrosine kinases often play pivotal roles in the proliferation and differentiation of many cell types. Many growth factor receptors contain a tyrosine kinase domain as part of their cytoplasmic tail such that binding by ligand directly activates their tyrosine kinase activity. However, many other receptors do not contain a tyrosine kinase domain in their cytoplasmic tail. Addition of ligand to many cell types expressing these receptors still results in increased levels of phosphotyrosine. The JAK family, a series of related intracellular tyrosine kinases, has recently been shown to link these receptors and other members of the signal transduction pathway.
The JAK family members contain the highly conserved catalytic domain found in other tyrosine kinases (Firmbach-Kraft, I., et al., Oncogene, 5:1329, 1990, Hanks, S. K., et al., Methods in Enzymology, 200: 38, 1991, Hunter, T., Methods in Enzymology, 200:3, 1991, Wilks, A. F., Proc. Natl. Acad. Sci. USA, 86:1603, 1989). One feature that distinguishes the JAK family from other tyrosine kinases is that each member also contains a second kinase-like domain of unknown function (Harpur, A. G., et al., Oncogene, 7:1347, 1992). In addition, the JAK family members do not contain SH2 or SH3 domains, signal peptide sequences, or transmembrane domains, and are localized in the cytoplasm (Wilks, A. F., et al., Molecular and Cellular Biology, 11:2057, 1991).
Three members of the JAK family, JAK1, JAK2, an TYK-2, have been functionally described. The first two members were isolated by a PCR approach utilizing degenerate oligonucleotide primers and TYK-2 was isolated by screening with a tyrosine kinase probe at reduced stringency (Silvennoinen, O. et al., Proc. Natl. Acad. Sci. USA, 90:8429,1993). To date, the JAK family members have been shown to be involved with the receptors for numerous cytokines and growth factors, including IFN .alpha..beta. and .gamma., IL-3, GM-CSF, EPO, GH, CNTF, LIF, OSM, IL-6, and PRL (Argetsinger, L. S., et al., Cell, 74:237, 1993, Luttichen, C., et al., Science, 263:89, 1994, Muller, M., et al., Nature, 366:129, 1993, Stahl, N., et al., Science, 263:92, 1994, Velazquez, L., et al., Cell, 70:313, 1992, Watling, D., et al., Nature, 366:166, 1993, Witthuhn, B. A., et al., Cell, 74:227, 1993, Rui, H., et al., The Journal of Biological Chemistry, 269:5364, 1994). In most cases, the JAK family members seem to associate with the proximal membrane portion of the cytoplasmic domain of the receptor (e.g., gp130, LIFR.beta., EPO) as a constitutive complex (Narazaki, M., et al., Proc. Natl. Acad. Sci. USA, 91:2285, 1994). In other cases, the association is not evident until ligand binding takes place (e.g., GH receptor). In either case, ligand binding results in increased JAK kinase activity.
The first evidence for the functional role of JAK family members was provided when it was shown that TYK-2 could rescue IFN.alpha./.beta. responsiveness in a cell line that had become unresponsive. In a similar fashion, JAK1 and JAK2 have been shown to function in the signalling of interferon pathways, as well. In each case, two different JAKS have been found to act with each type of IFN receptor; JAK2 and TYK-2 are involved exclusively with IFN .gamma. and IFN .alpha./.beta., respectively, whereas JAK1 is involved with both receptors. Stimulation of the IFN.alpha./.beta. receptors by the binding of their respective ligands results in the phosphorylation of p91 (STAT1) and p113 (STAT2), which are subunits of the ISGF3 transcription complex that binds the interferon-stimulated response element (ISRE). In the case of IFN.gamma., p91 alone is phosphorylated, which then binds gamma-activated sequences (GAS) of IFN.gamma. activated genes (Shual, K., et al., Nature, 366:580, 1993, Ihle, J. N., et al., Trends in Biological Science, 19:222, 1994). Because each of these receptors associate with JAK1 it has been suggested that JAK1 may directly phosphorylate p91 (Loh, J. E., et al., Molecular and Cellular Biology, 14:2170, 1994). It has been recently reported that IL-6 (via gp130), which associates with JAK1 and TYK-2, also triggers the activation of p91 (STAT1) (Yuan, J., et al., Molecular and Cellular Biology, 14:1657, 1994). The EPO, and IL-3 receptors are also believed to similarly activate STAT family members. As all of the hematopoietic receptors seem to utilize certain common proteins in their signal transduction pathways, some of the specificity of the pathways may reside in the cell specific expression of STAT family members and their activation by JAK family members (Metcalf, D., Blood, 82:3515, 1993, Darnell, J. E., et al., Science, 264:1415, 1994).
Additional pairs of JAK family members have been found to associate with other receptors (e.g., CNTF, LIF, IL-6) and both become tyrosine phosphorylated upon the stimulation of these receptors (Silvennoinen, O., et al., Nature, 366:583, 1993). It is possible that reciprocal tyrosine phosphorylation between two JAKs is required as phosphorylation of both associating JAKs is necessary for signal transduction to occur. Thus, JAK family members may act in pairs, possibly as heterodimers.
Recently a Drosophila JAK family member, hop, was shown to be required maternally for normal embryonic development (Binari, et al., Genes & Dev., 8:300, 1994). Mutants in hop showed defects in the expression of several paired-rule and segment polarity genes, implicating it in the control of transcription of these genes, a role that could be analogous to the defect in TYK-2, JAK1, or JAK2 in several cell lines that lost IFN responsiveness.
The present invention provides a new member of the JAK protein tyrosine kinase family. The structural homology between the JAK3 of this invention and the other members of JAK family, indicates that JAK3 is a new member of this family of non-receptor tyrosine kinases. In analogy to the other JAK family members, JAK3 is likely involved in the signal transduction pathway of already characterized receptors which lack intrinsic activity. Because of its strong expression in the fraction enriched for CD34+ normal human bone marrow, JAK3 is likely to be important in stem/progenitor cell growth and/or differentiation, by transducing the signals of receptors which modulate these processes. In addition, JAK3 may also be involved in the signal transduction pathways of any of several non-tyrosine kinase receptors with which the other JAK members have not been shown to associate (e.g. IL-2, IL-4, IL-11).